I was going to put this in with the "Most Chaotic 1960's" thread, but I decided to branch it off.
In 1971, the Weather Underground exploded a bomb in the ground floor of the US Capitol building. In 1972, the Weather Underground exploded a bomb in the fourth floor women's bathroom of the Pentagon. I don't think either one caused more than some structural damage.
The question therefore becomes what if these D.C. bombings were worse? IE, they caused the deaths of government officials (Senators, Congressmen), and military officials, and caused major structural damage, or were expanded to other government structures and D.C. buildings in a terror campaign? Logistically, is it possible? And what would be the effects?